The present invention relates to centrifugal pumps in general, especially to multi-stage volute type centrifugal pumps, and more particularly to improvements in inducers for use in centrifugal pumps. Still more particularly, the invention relates to improvements in propeller type inducers for use in centrifugal pumps.
The inducer of a centrifugal pump serves as a means for feeding fluid (e.g., water or another liquid) to the adjacent impeller. The output of such inducers is normally selected in such a way that they can achieve the net positive suction head of the associated impeller at a predetermined operating point. In accordance with the criteria on which the operation of a centrifugal pump with an inducer is based, the inducer should operate in the partial load range (i.e., at less than full capacity) at any operating point of the pump. As a rule, the inducer is a propeller (i.e., it induces axial flow of fluid). A drawback of conventional inducers is that, when they are operated in the partial load range, they invariably generate turbulence (flow coming out of the impeller toward the inducer) which results in noise, pulsating fluid flow and unstable operation of the pump.
Attempts to overcome the drawbacks of conventional porpeller-type inducers include the placing of an apertured disk in front of the inducer. The disk is intended to reduce pulsations of the fluid stream and to reduce the space which is available for development of back-flow turbulence. It has been found that the disk is incapable of appreciably reducing the noise because noise develops as a result of turbulence and the apertured disk merely reduces or confines but cannot eliminate turbulence.
German offenlagungsschrift Pat. No. 1,528,836 discloses an inducer with a hub whose diameter increases in the direction of fluid flow. The hub carries a circumferentially extending blade. The purpose of the specially designed hub is to fill the region in which the reflux or back-flow turbulence develops. Such types of inducers reduce turbulence at the rated point of the pump; however, they are much less effective in the partial load range.